Several techniques and systems have been developed for correcting and stabilizing the spine and for facilitating fusion at various levels of the spine. Stabilization of the spine for various conditions, including degenerative disk disease, scoliosis, spondylolisthesis, and spinal stenosis, often require attaching implants to the spine and then securing the implants to spinal rods. Such spinal fixation devices can immobilize the vertebrae of the spine and can alter the alignment of the spine over a large number of vertebrae by connecting at least one elongate rod to the sequence of selected vertebrae. These rods can span a large number of vertebrae, such as three or four. The spine anatomy, however, rarely allows for three or more implants to be directly in line. In order to allow for this irregularity, the rod must be contoured to the coronal plane.
Spinal fixation has become a common approach in fusion of vertebrae and treating fractures and the above listed spinal disorders. A common device used for spinal fixation is a bone fixation plate assembly. Typical bone fixation plate assemblies have a relatively flat, rectangular plate with a plurality of apertures therethrough. Another option is an implantation fixation system that locks a rod to several vertebrae. In these system, as with other spinal fixation systems, various fasteners, such as bone screws, are used to secure the implantation fixation assembly to the desired and targeted vertebrae of the patient. These screws vary in design and shape depending upon their desired location and use.
Polyaxial locking screws are frequently used as fasteners in implantation fixation systems. Once these screws are set in a desired position, the screws must be securely fixed in that position. Movement of the screw must be minimized or eliminated. This requires a fixation system that securely engages the polyaxial screw and minimizes or prevents movement of the screw.
There are numerous polyaxial screws and fixation systems existing in the market today. For example, U.S. Pat. No. 6,740,086 to Richelsoph (the '086 Patent) discloses a fixation system having a hollow fixing mechanism. The fixing mechanism has a central passage and a polyaxial screw inserted into the central passage. In a preferred embodiment, the screw has a head portion that seats inside one end of the hollow fixing mechanism, and a threaded shank that projects through the end of the fixing mechanism in an exposed manner. The fixing mechanism has wall portions that form channels for receiving a rod. The rod is seated in the channels and extends transversely through the central passage. The wall portions include internal threads along the passage for receiving a set screw. The set screw has an external thread that engages the internal threads in the passage. In this arrangement, the set screw can be threaded into the passage and advanced into the passage into engagement with a rod or rod seating mechanism to securely fix the polyaxial screw head to the fixing mechanism and rod.
The fixation system of the '086 Patent utilizes a threaded engagement, which provides a mechanical advantage analogous to an inclined plane. The threads provide a means of overcoming a large amount or axial resistance imparted by the rod. The mechanical advantage of the inclined plane creates large axial forces that compress the screw head and fixing mechanism together, forming a rigid construct. One drawback of this construct, and of all systems that utilize set screws or other rotating locking elements, is the requirement of torque to tighten the locking element. When the locking element is tightened through torque, the torque gradually transfers to the fixing mechanism and polyaxial screw. A significant amount of torque is typically applied in the final tightening. This introduces a substantial risk of “blowout”, in which torque and/or other components of force tilt the shank out of its set alignment in the screw hole, causing the shank to break through the relatively thin bone wall of the pedicle. In such a case, removal and resetting of the polyaxial screw can exacerbate the trauma to the bone.
To control the risk of blowout, some practitioners use additional instrumentation to apply a countertorque to the fixation mechanism, so that the torque applied to set screw does not cause rotation or displacement of the fixing mechanism and polyaxial screw. This requires the careful balancing of torque with countertorque, and any imbalance can still cause blowout. Moreover, application of countertorque requires an additional instrument to be used at the same time that the set screw is being driven into the fixing mechanism. Aside from the obvious disadvantage of adding to instrument costs and instrument preparation, the countertorque instrument can be cumbersome to use while advancing the locking element at the same time. A surgeon who advances the set screw and holds the countertorque instrument at the same time will not have any hands free. This may compel the need for additional medical personnel during installation of the fixing assembly. The countertorque instrument further adds to the visual obstructions over the incision, and may require the size of the incision to be made larger to accommodate the additional instrumentation.
Systems that use threaded locking mechanisms are also difficult to use, requiring precise coordination and mating of components. In many cases, the assembly is very small, and proper thread starting can be difficult. If the threading is not started properly, the locking mechanism can bind, damaging the threaded surfaces and rendering the components unusable.
Accordingly, there is a need for a fixation mechanism or device that provides a strong and secure locking of the screw in its desired position, with minimal risk to the impacted bone area. Additionally, there is a need for a fixation mechanism or device that is not cumbersome to operate, and requires less instrumentation. Furthermore, there is a need for a fixation mechanism or device that more efficiently establishes a secure locking of the screw, with less time and effort required than prior systems and techniques.